Personal visual display

ABSTRACT

A visual display device is provided for delivering a generated image, preferably combinable with environment light, to the eye of a user. Light from an image generator such as a CRT, LED or LCD is reflected from a fold mirror away from a user&#39;s eye towards a combiner. The image is reflected from the combiner and magnified, optionally combined with light from the environment and passes back through the fold mirror towards the eye of the user. In one embodiment, an optical element such as a meniscus lens or PCX lens is used to present the user with a substantially flat focal field and/or a Fresnel lens or diffractive optical element is used to aim rays from the image to the field curvature corrective lens in order to maintain maximum contrast.

This application is a continuation-in-part of Ser. No. 08/307,669, filedApr. 21, 1995, now U.S. Pat. No. 5,864,326 which is the U.S. NationalPhase of PCT/US94/01390, filed Feb. 7, 1994, for "DEPIXELATED VISUALDISPLAY" and is further a continuation-in-part of Ser. No. 08/150,966filed Nov. 12, 1993, now abandoned, for "OPTICALLY CORRECTED ELEMENTMOUNTED DISPLAY" which is a divisional of Ser. No. 07/832,237, filedFeb. 7, 1992, now U.S. Pat. No. 5,303,085.

This invention relates to visual displays which preferably can combinegenerated images with a view of the environment surrounding a user andtransmit such combined visual information to the eye of the user.

BACKGROUND OF THE INVENTION

It is often desirable to provide visual information to a living being.Frequently, one wishes to superimpose such visual information upon thebeing's view of the real world. In other applications, it is desired toshield the user from a view of the environment, providing an exclusiveview of the visual information.

Such displays include a number of components including, in a form knownas a folded catadioptric head-mounted display, an image generator, abeam splitting fold mirror which receives the image light from the imagegenerator, often via a series of lenses or other optical elements, andsends a fraction, designated the reflected fraction, of such image lightto a reflective combiner that is either non-transmissive or both allowslight from the real world to pass through such combiner and reflects theimage light such that both the real-world light and the image light aretransmitted through the beam splitter to the eye of the user, often viaanother series of lenses or other optical elements. The beam splitterwill transmit a fraction, designated the transmitted fraction, of theimage light reflected from the combiner. In embodiments in which thecombiner is at least partially transmissive, a fraction of thereal-world light is also transmitted by the beam splitter.

Previous devices included a number of additional components. Somedevices have included corrective optical elements. Other devices haveincluded a depixelator, e.g., as described in PCT/US94/01390 filed Feb.7, 1994 for "Depixelated Visual Display" (incorporated herein byreference). Still other devices have included apparatus for intensifyingthe visual display such as those described in PCT/US94/01391, filed Feb.7, 1994 for "Intensified Visual Display" (incorporated herein byreference).

In general, there are certain disadvantages to including componentsother than an image generator, beam splitter and reflective combiner. Inmost cases, each such additional item will contribute to the increasedweight of the display device. When the display device is intended to bemounted on the user's head, such additional weight can contribute todiscomfort and detract from the ability to move the head quickly andnaturally as is desired for virtual reality applications. Furthermore,such additional devices typically must be correctly and preciselyaligned and the difficulty and burden of alignment, both during theoriginal manufacture and during any repair or maintenance that may beneeded, is compounded by the number of additional components which areincluded in the device. Furthermore, both the optical design and themechanical design become complicated when such additional components areincluded. Moreover, additional components typically increase the cost ofthe final device.

Accordingly, it would be useful to provide a visual display apparatuswhich has a reduced or minimized number of components and yet provides avisual display transmitted to the eye of the user having a desirablyhigh quality and intensity of image.

SUMMARY OF THE INVENTION

One embodiment of the invention is a head-mounted display (HMD) that canproject an image from an image generator such as a cathode ray tube(CRT) or a liquid crystal display (LCD) to one of each of the eyes ofthe observer or both transmit such an image and combine it with a directview of the surrounding environment. The combiner images a CRT or LCDdisplay surface mounted above the eye with a simple metallic, dielectricor holographic fold mirror reflecting the image towards the combiner.

According to one aspect of the present invention, the device simplifiesthe imaging optics by reducing the number of elements to three or fewer.One embodiment of the invention consists of an image generator such as aCRT or LCD, a combiner such as a spherical metallic, dielectric orholographically made combiner or collimator-combiner and a fold mirrorsuch as a metallic, dielectric or holographically made fold mirror.

In one embodiment, no additional optical elements are needed to achievethe desired high quality image. Preferably, the visual display issimplified and made lightweight and less expensive by eliminating theneed for certain optic elements such as relay lenses or additionalcorrective refractive optics. In one embodiment, a visual displayprovides for monochromatic and/or three-color display without refractiveoptics, such as by using reflective imaging optics only. In anotherembodiment, a single additional optical element is provided. In oneembodiment, the additional optical element is provided to accommodatethe curved reflecting surface of the combiner and provide an essentiallyflat focal field to the user. In another embodiment, a depixelatorand/or an image intensifier can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational schematic view of a visual display deviceaccording to one embodiment of the present invention;

FIG. 2 is a side elevational schematic view of a visual displayaccording to one embodiment of the present invention;

FIG. 3 is a perspective view, showing a head-mounted display deviceaccording to one embodiment of the present invention;

FIG. 4 is a side view, partially cut away, showing a head-mounteddisplay device according to one embodiment of the present invention;

FIGS. 5A and 5B are side schematic views showing effective viewingangles in the absence of contrast correction; and

FIG. 6 is a side schematic view showing corrected viewing angles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the visual display comprises, as shown in FIG. 1, animage generator 2, a fold mirror 1 positioned to receive the image lightfrom the image generator 2 and to reflect such image light and areflective combiner 4 situated to receive the image light from the foldmirror 1, reflect such image light, which may be combined with lightrays transmitted through the reflective combiner 4 from the ambientenvironment and transmit the combined environmental light rays and imagelight through the fold mirror 1 to the eye 5 of the user. An opticalpath 6 of the image light from the image generator 2 to the eye 5 isdepicted in FIG. 1. Preferably the present invention provides asubstantially on-axis configuration. In the on-axis configuration of anembodiment of the present invention, the image light from the imagegenerator impinges the fold mirror at an angle 102 of about 45°(preferably, plus or minus about 5°) and/or the tangent plane 104 at thecentroid 106 of the combiner 4 is at an angle 108 of about 90°(preferably, plus or minus about 5°) with respect to the output plane100 of the image generator 2. An on-axis system avoids a keystonedistortion in the image (and thus avoids the requirement for additionaloptics if the distortion is to be corrected). In one embodiment, theimage light passes substantially directly (i.e., without passing througha intervening optical element) from the image generator to the foldmirror and/or from the fold mirror to the combiner.

An embodiment illustrated in FIG. 2, includes an optical element, suchas a field curvature correction lens 7, placed between the imagegenerator 2 and fold mirror 1. In the depiction of FIG. 2, the lens 7 isa plano-convex lens with its flat surface oriented toward the imageplane of the image generator. The various components of the depictedembodiments will be described below in greater detail. Preferably,separate images are generated and transmitted to each eye of the, userso that, in one embodiment, there will be two sets of components,depicted in the FIGS. 1 or 2, one for each eye.

The image generator 2 can be any of a plurality of imagining devicesincluding a cathode-ray tube (CRT), light emitting diode (LED) array,liquid crystal display (LCD), field emissive device (FED), monochromeimage source such as a monochrome CRT, which may be used in connectionwith a liquid crystal color shutter, as well as laser devices such aslaser diode arrays. Preferably, an LCD device includes the capabilityfor being backlit using any of a number of backlighting devices (notshown) such as incandescent lighting, fluorescent lighting,electro-luminescent and ambient lighting. The image generator can be amonochrome or color image source. In one embodiment, a colorizationelement such as a liquid crystal color shutter can be used, e.g., toconvert a monochrome image into a frame sequential color image. A colorshutter system is provided, for example, in Tektronix EX100HD 1 inchColor Display System. In one embodiment, a color LCD such as the SeikoEpson FO7KM200, 138,000 pixel, 0.7" diagonal, backlit LCD can be used,although other devices are also operable. The image generator 2generates an image under control of an image source 10 communicatingwith the image generator over a communication link 12. The imagegenerator can receive image information from sources such as a computer,video cassette recorder (VCR), laser disc, video camera and the like.The communication link can be a cable, optical fiber or can be awireless link, such as an infrared or a radio communication link. In oneembodiment, the image generator 2 receives image information from aportable computer, running 3-dimensionally displayed softwarecommunicating over a cable link.

In the depicted embodiment, the image generator 2 is mountedsubstantially above the fold mirror 1 (meaning vertically above, whenthe device is oriented so that the axis between the eye position 5 andcombiner 4 is substantially horizontal). With reference to thisconfiguration, the image light travels downward from the image generator2 towards the fold mirror 1. The image light is at least partiallyreflected by the fold mirror 1 in a substantially horizontal directionaway from the eye of the user 5 and towards the combiner 4. At thecombiner 4, the image light is at least partially reflected back towardsthe fold mirror 1. In an embodiment in which the user willsimultaneously view the image or the environmental light, light from theenvironment passes through the substantially transparent combiner 4 andtravels along with the image light reflected from the combiner 4 toproduce combined light. In an embodiment in which the user will viewonly the generated image, without being able to view the environment(referred to as an "immersive" device), the combiner is substantiallyfully reflective and substantially non-transmissive (so that, in thiscontext, "combiner" is used only for convenience). The image lightand/or the combined light is at least partially transmitted through thefold mirror 1 towards the eye of the user 5. The amount of light, ifany, which passes from the surrounding environment to the eyes of theobserver can be at least partially controlled by the coatings placed onthe surfaces of the combiner and the fold mirror.

A number of technologies can be used to form the fold mirror 1 and thecombiner 4. In some devices, holographic optical elements (HOEs) can beused to make the fold mirror and/or the combiner. Various holographicand non-holographic techniques can be used for these purposes, asdescribed more fully below and, as described, for example, in U.S.patent application 08/150,966 filed Nov. 12, 1993 for "OpticallyCorrected Helmet Mounted Display" and the parent application thereof,Ser. No. 07/832,237, filed Feb. 7, 1992, now U.S. Pat. No. 5,303,085,both of which are incorporated herein by reference.

The fold mirror 1 is a flat, partial reflector. In the depictedembodiment, it is positioned at approximately 45° with respect to theoptical path of the image light 6. The fold mirror reflects theprojected image from the image source in a direction away from the eye 5and towards the combiner. The fold mirror also allows at least a portionof the light reflected from the combiner to pass through the fold mirrorto the eye.

Preferably, the fold mirror is configured to reflect light evenly acrossthe visible spectrum, (approximately 450 to 650 nanometer wavelengths).In one embodiment, the fold mirror is a 50% splitter (reflecting 50% oflight and passing 50% of light). It can be made holographically,dielectrically or metallically. The fold mirror can be formed usingglass or plastic substrates, as well as other substantially transparentmaterials such as crystalline material, polycrystalline material,transparent ceramics and the like. The surface of the fold mirror whichfaces the image generator is, typically, the reflective surface. Thesurface which is opposite the reflective surface preferably has ananti-reflective coating. The anti-reflective coating is useful inpreventing a ghosting reflection from that surface. Holographic anddielectric fold mirrors can be made to selectively reflect differentpolarizations at higher rates, and when used with wave retardationmaterials can achieve very high throughput efficiencies, such asdescribed in patent application PCT/US94/01391.

In one embodiment, the fold mirror cap be produced either from vapordeposition of metallic or dielectric materials on the reflective side,and an anti-reflective coating such as Magnesium Fluoride using the samevapor deposition process. It can also be manufactured using holographicphotopolymers or dichromatic gelatin (DCG) for the reflective surface.The anti-reflective surface is vapor deposited onto the back surface ofthe fold mirror.

The combiner 4 is a fully or partially reflective optical element. Inone embodiment, it is spherically or aspherically curved preferablyhaving a radius of curvature between about 50 mm and about 80 mm, morepreferably between about 55 mm and about 70 mm and most preferably about67 mm. Preferably, the combiner 4 serves as the primary magnificationelement for the image light. An optical element is referred to as beinga "powered" element if it provides image magnification. Although, forconvenience, this item is referred to as a "combiner", in someembodiments the combiner may be used without providing for a combinationof image light with environmental light, as described more fully below.The combiner can be formed on a substrate of glass, plastic, or othertransparent or partially transparent material or, when isolation fromenvironmental light is desired, from opaque or substantially opaquesubstrates such as metal, non-transparent ceramics, plastics or glassand the like.

In one embodiment, the combiner can be made holographically usingphotopolymers, dichromated gelatin (DCG) or other similar holographicmaterials. As is well known in the art, holographic elements can befinely tuned to reflect specific bands of wavelengths and transmit otherwavelengths. In one embodiment, the combiner is tuned to achieve highreflection of red, green and blue wavelengths which are the wavelengthsoutput by the image generator (and which, when combined, produce fullcolor images). In one embodiment, the holographic combiner is a singleline or multi-line holographic device, which can be tuned to reflect oneor more wavelengths. For example, a three-line holographic device can betuned to reflect wavelengths of, e.g., approximately 480, 545 and 630nanometers (plus or minus about 20 nanometers). Other wavelengths, suchas wavelengths from the environment other than those in a narrow rangearound 480, 545 and 630 nanometers, will be transmitted through thecombiner, towards the eye of the user with a high degree oftransmissivity. Depending on the application, the hologram can beconfigured to reflect more, fewer or different wavelengths. Althoughdigital holography is an operable technique, it is currently believedthat this technique, which can be expensive to implement, is not neededto achieve the high-quality image of the present invention, andaccordingly, a traditional (non-digital or non-synthetic) technique ispresently preferred, when holograms are to be used for the combiner.

Combiner can also be made using broadband dielectric technology. Thesecombiners are made through the deposition of successive layers ofdielectric material such as silicon dioxide and titanium dioxide. Thistype of coating can be provided so as to evenly reflect across thevisible spectrum, such as evenly reflecting wavelengths betweenapproximately 450 nanometers and 650 nanometers. Such evenness ofreflection is particularly useful in maintaining color integrity of theimage. Combiners made with a broadband dielectric coating are lessefficient and less transmissive than holographic combiners and thus areuseful in applications where high transmissivity is not required (suchas virtual reality applications in which environmental light is notdesired). Fully or almost fully immersive dielectric combiners can bemade by dipping the combiner substrate into a light absorbing dye. Thedying process can be altered to produce combiners of a given desiredtransmissivity specification, for example, about 1%.

A combiner can also be formed on a substrate using an aluminum or othermetallic coating in order to achieve the desired reflectivity of thecombiner. These materials are relatively inexpensive compared withholographic or dielectric combiners but are not as efficient. Thesecombiners are easily made to be fully immersive and, by decreasing thethickness of the coating, can be made to be transmissive as well.

In one embodiment, the magnification provided by the combiner 4 isconfigured so that center of the image from the image generator appearsto the user, to have an infinite focus, (i.e., appears to be in focus tothe user when the user is focusing his or her eyes at a far, essentiallyinfinite, distance). In many configurations, portions of the image whichare not at the center have a focus, perceived by the user, at some pointcloser than that at the center of the image. Typically, this differencewill increase as one progresses farther from the center of the generatedimage. In many applications this difference in perceived focal distanceis undesirable since, e.g., it forces the user to constantly shift hisor her eye focus when viewing different portions of the generated image.

In some embodiments it may be preferable to accept a certain amount ofdifference in focal distance across the generated image, e.g., in orderto reduce or minimize the total weight of the visual display device, thecost, complexity and the like. In some applications, however, it isdesired to include an additional optical element in order to correctthis optical characteristic and present the user with a substantiallyflat focal field for the generated image. Embodiments of the inventioncan be used to reduce the aberrations and distortions produced by thespherical combiner such as image field curvature. The embodiment of FIG.2 depicts one approach for achieving this correction. In theseembodiments, additional optical element 7 is positioned somewhere alongthe optical path between the image generator and the eye of the user. Inthe embodiment of FIG. 2, it is placed between the image generator 2 andthe fold mirror 1. In the embodiment depicted in FIG. 2, the additionaloptical element is a plano-convex (PCX) lens.

In another embodiment, a meniscus lens, e.g., a negative meniscus lenscan be used to simultaneously correct field curvature and to aim theimage light in a more favorable direction. Aiming the image light can beuseful for a number of purposes, including maintaining high contrastimages, as described below. Image rays exiting from the image generatornear normal (90°± about 10° with respect to the image plane) often havethe highest contrast ratio.

As depicted in FIG. 5A, in configurations which include only an imagegenerator 2, fold mirror 1 and combiner 4, while the effective viewingangle 32 near the center of the image will be relatively close to normal(i.e., 90° to the image output plane), typically departing therefrom byno more than about 10°. However, portions of the image generated nearthe edges of the image generator 2 have an effective viewing angle 34which is larger, and, in many cases, will be large enough (such as about10° or more from normal) to have a degenerated or even reverse contrast.This typically presents the greatest problem when the image generator 2is a liquid crystal display since LCDs are often subject to loss ofcontrast at high viewing angles. This manifests itself in a perceptionof a low contrast or a reverse contrast image near the periphery or edgeof the image. The problem of loss of contrast near the edge of the imageis even more acute when a plano-convex lens 7 is included as depicted inFIG. 5B or when the image generator 2 has a relatively larger size suchas having a diagonal size of 1 inch or more (although some loss ofcontrast of the periphery may still occur for smaller image generatorssuch as 0.7 inch diagonal image generators).

According to one embodiment of the present invention, loss of contrastcan be reduced or eliminated by providing an optical element whichaccepts light rays at normal incidence from the LCD and redirects thelight so as to enter the field curvature correction lens (where present)at the required angles and so as to produce the desired magnificationand/or field curvature correction. FIG. 6 depicts one such apparatuswhich provides a negative meniscus lens 36, preferably adjacent and/orcontacting the output plane of the image generator 2. Preferably, thenegative meniscus lens is formed by an upper surface 37 of device 36while a lower surface 37' acts as the field curvature correction lens.As can be seen from FIG. 6 the effect of the negative meniscus lens isto provide an effective viewing angle for all portions of the imagegenerated by the image generator 2 which is a relatively low angle (withrespect to the normal direction from the output plane of the imagegenerator) such as less than about 10°, preferably less than about 5°and more preferably less than about 3° from normal. Other opticalelements which can achieve this manipulation are diffractive opticalelements (DOEs), fresnel lenses and other refractive optical elements.When a diffractive optical element is used it can be placed directly onthe LCD surface or on the field curvature correction lens or on aseparate element, e.g., located between the image generator 2 and thefield curvature correction lens. Such a diffractive optical elementwould receive light rays at normal incidence from the image generator 2and redirect the rays through diffraction, to the proper angles requiredby the plano-convex lens 7 used for field curvature correction.

Aiming the image rays is achieved using a diffractive optical element(such as a zone plate or hologram) or a Fresnel optic acting as anegative lens to accept collimated light, at substantially normalincidence from the image source, redirect it into a divergent beam whichis then recollimated by the field curvature correction lens.

FIGS. 3 and 4 depict the use of the elements of FIG. 2 in a head mounteddisplay. In one embodiment, the optics are fully or partially protectedby being enclosed in a shroud 12, preferably with separate shrouds 12aand 12b for each eye as described in PCT application PCT/US94/09819 forPersonal Visual Display System, filed on an even date herewith andincorporated herein by reference. A covering such as visor top 14further shields from stray light and protects the optical elements fromdust or other contamination. In the depicted embodiment, the device isheld in position by strap 16 and a forehead brace 18.

In view of the above description, a number of advantages of the presentinvention can be seen. The present invention provides for a high qualityvisual display which can combine a generated image with environmentallight, but which is nevertheless lightweight and of reduced complexityand cost. In one embodiment, the generated image is magnified but isalso presented to the user with a substantially flat focal field.

A number of variations and modifications can also be used. Although theinvention provides for high quality image and a lightweight device atleast partially by reducing or minimizing the number of components,certain additional optical components can be used if desired such asimage intensifiers, depixelators and diffractive optical elements andFresnel optics.

When a depixelator is used, it can be provided as a crossed diffractiongrating located parallel to the pixel plane of an image generator toreceive, diffract, and transmit and thereby depixelate the light imagefrom the image generator. Preferably, the spatial frequency of thecrossed diffraction grating is constructed such that this spatialfrequency multiplied by the shortest wavelength of the image light thatis used approximately equals the center-to-center distance betweenadjacent pixels in the pixel plane of the image generator divided bytwice the optical distance between the crossed diffraction grating andthe pixel plane of the image generator.

It is preferable that the modulation depth of the crossed diffractiongrating fall within the range of 80% to 120% and that the crosseddiffraction grating be sinusoidal or slightly squared.

When the additional optical component is an image intensifier, theintensifier can be provided as follows. If the light from the imagegenerator is S polarized, a phase retarder, such as a quarter-wave plateor a polarization rotator, such as a liquid crystal device is positionedso that the S polarized image light reflected from the fold mirror istransmitted through e.g., a quarter-wave plate and retarded byone-quarter wave in order to exit the quarter-wave plate as circularlypolarized image light. The reflective combiner is situated to receivethe circularly polarized image light from the quarter-wave plate,reflect such circularly polarized image light, combine such circularlypolarized image light with light rays transmitted through saidreflective combiner from the surrounding environment, and transmit thecombined environmental light rays and circularly polarized image lightthrough the quarter-wave plate, which again retards the circularlypolarized image light by one-quarter wave so that it exits thequarter-wave plate as P polarized light which then passes through thefold mirror with the environmental light rays and subsequently reachesthe eye position of the user, as also do the environmental light rayswith which the image light has been combined. Optionally, thequarter-wave plate is attached to the reflective combiner.

The precise nature of the quarter-wave plate is not critical. It is,however, preferable to employ a single-order plate so that dependence onangle and wavelength is minimized. Quarter-wave plates are made fromplastics and crystals or constructed holographically. A plastic waveplate is, however, more dispersive, and has lower optical propertiesthan a crystalline or holographic wave plate.

If the light from the image generator is P polarized, the user-mounteddisplay comprises a first phase retarder such as a quarter-wave plate ora polarization rotator, located so that the P polarized image light froman image generator is transmitted through said first quarter-wave plateand retarded by one-quarter wave in order to exit the first quarter-waveplate as circularly polarized image light; a second quarter-wave platepositioned so that the circularly polarized image light from the firstquarter-wave plate is transmitted through said second quarter-wave plateand retarded by one quarter-wave in order to exit the secondquarter-wave plate as S polarized image light. The fold mirror receivesthe S polarized image light from the second quarter-wave plate andreflects such S polarized image light through the second quarter-waveplate where the S polarized image light will again be retarded byone-quarter wave and, consequently, exit said second quarter-wave plateas circularly polarized image light. The reflective combiner receivesthe circularly polarized image light from the second quarter-wave plate,reflects such circularly polarized image light, combines such circularlypolarized image light, combines such circularly polarized image lightwith light rays transmitted through the combiner from the surroundingenvironment, and transmits the combined environmental light rays andcircularly polarized image light through the second quarter-wave plate,which again retards the circularly polarized image light by one-quarterwave so that it exits the second quarter-wave plate as P polarized imagelight which then passes through the fold mirror with the environmentallight rays and subsequently reaches the eye position of the user, asalso do the environmental light rays with which the image light has beencombined.

The second quarter-wave plate can optionally be laminated to the foldmirror. This has the advantage of reducing the number of componentswhich must be optically aligned. Additionally, it creates fewer surfaceson which an anti-reflective coating must be placed.

Although the depicted configuration is an on-axis system, the presentinvention can also be used in connection with an off-axis system.Although the depicted embodiment involves a binocular or biocularconfiguration a monocular configuration can also be used. Although thedepicted embodiment shows a head-mounted display, other mountingtechniques can be used such as a helmet mounted, fixed-eyepiece displaysor the like.

Although the present invention has been described by way of a preferredembodiment and certain variations and modifications, other variationsand modifications can also be used, the invention being defined by thefollowing claims.

What is claimed is:
 1. Head-mounted visual display apparatus for a user,comprising:an image generator which outputs image light, said imagegenerator having a substantially planar output surface to output saidimage light in an image plane, said planar output surface beingnon-orthogonal to the plane of the user's eyes, wherein wavelengths ofsaid image light are from approximately 480 nanometers to approximately630 nanometers; a fold mirror configured to reflect light evenly acrossthe visible spectrum, positioned to receive the image light from theimage generator and to reflect at least a first portion of said imagelight in a first direction; a reflective combiner situated to receivesaid first portion of said image light from said fold mirror, reflect atleast a second portion of said image light, combined with environmentallight transmitted through the reflective combiner to produce combinedlight, wherein at least some of said combined light is transmittedthrough said fold mirror in a second direction to the eye position ofthe user, providing an optical path from said image generator to saidfold mirror to said combiner, back through said fold mirror and to saideye position of the user; said image generator and said reflectivecombiner being mounted on the head of said user at a level below the topof the head of said user; wherein said visual display apparatus isprovided in the absence of any optically active components between saidimage generator and said fold mirror.
 2. An apparatus, as claimed inclaim 1, wherein said image light includes red, green and blue light. 3.Head-mounted visual display apparatus for a user, comprising:an imagegenerator which outputs image light, said image generator having asubstantially planar output surface to output said image light in animage plane, said planar output surface being non-orthogonal to theplane of the user's eyes; a fold mirror configured to reflect lightevenly across the visible spectrum, positioned to receive the imagelight from the image generator and to reflect at least a first portionof said image light in a first direction; a reflective combiner situatedto receive said first portion of said image light from said fold mirror,reflect at least a second portion of said image light, combined withenvironmental light transmitted through the reflective combiner toproduce combined light, wherein at least some of said combined light istransmitted through said fold mirror in a second direction to the eyeposition of the user, providing an optical path from said imagegenerator to said fold mirror to said combiner, back through said foldmirror and to said eye position of the user; said image generator andsaid reflective combiner being mounted on the head of said user at alevel below the top of the head of said user; an optical element,distinct from said combiner and said fold mirror, positioned betweensaid fold mirror and said combiner.
 4. Apparatus, as claimed in claim 3,wherein said optical element positioned between said fold mirror andsaid combiner comprises a phase retarder.
 5. An apparatus, as claimed inclaim 3, wherein said optical element is positioned for receiving saidimage light from said image generator prior to said image lightcontacting said fold mirror.
 6. Head-mounted visual display apparatusfor a user, comprising:an image generator which outputs image light,said image generator having a substantially planar output surface tooutput said image light in an image plane, said planar output surfacebeing non-orthogonal to the plane of the user's eyes; a fold mirrorconfigured to reflect light evenly across the visible spectrum,positioned to receive the image light from the image generator and toreflect at least a first portion of said image light in a firstdirection; a reflective combiner situated to receive said first portionof said image light from said fold mirror, reflect at least a secondportion of said image light, combined with environmental lighttransmitted through the reflective combiner to produce combined light,wherein at least some of said combined light is transmitted through saidfold mirror in a second direction to the eye position of the user,providing an optical path from said image generator to said fold mirrorto said combiner, back through said fold mirror and to said eye positionof the user; said image generator and said reflective combiner beingmounted on the head of said user at a level below the top of the head ofsaid user; a refractive optical element located between said imagegenerator and the fold mirror wherein said refractive optical elementavoids a reduction in contrast in the image.
 7. Head-mounted visualdisplay apparatus for a user, comprising:an image generator whichoutputs image light, said image generator having a substantially planaroutput surface to output said image light in an image plane, said planaroutput surface being non-orthogonal to the plane of the user's eyes; afold mirror configured to reflect light evenly across the visiblespectrum, positioned to receive the image light from the image generatorand to reflect at least a first portion of said image light in a firstdirection; a reflective combiner situated to receive said first portionof said image light from said fold mirror, reflect at least a secondportion of said image light, combined with environmental lighttransmitted through the reflective combiner to produce combined light,wherein at least some of said combined light is transmitted through saidfold mirror in a second direction to the eve position of the user,providing an optical path from said image generator to said fold mirrorto said combiner, back through said fold mirror and to said eye positionof the user; said image generator and said reflective combiner beingmounted on the head of said user at a level below the top of the head ofsaid user; wherein one of a phase retarder and a polarization rotatingdevice is positioned along the optical path.
 8. Apparatus, as claimed inclaim 7, wherein said phase retarder is a quarter-wave plate. 9.Head-mounted visual display apparatus for a user, comprising:an imagegenerator which outputs image light, said image generator having asubstantially planar output surface to output said image light in animage plane, said planar output surface being non-orthogonal to theplane of the user's eyes; a fold mirror configured to reflect lightevenly across the visible spectrum positioned to receive the image lightfrom the image generator and to reflect at least a first portion of saidimage light in a first direction; a second mirror situated to receivesaid first portion of said image light from said fold mirror, reflect atleast a second portion of said image light, wherein at least some ofsaid image light is transmitted through said fold mirror in a seconddirection to the eve position of the user, providing an optical pathfrom said image generator to said fold mirror to said second mirror tosaid eve position of the user; said image generator and said secondmirror being mounted on the head of said user at a level below the topof the head of said user; wherein said second mirror is substantiallynon-transmissive.
 10. Visual display apparatus for a user, comprising:animage generator having a first output surface, which outputs imagelight; a fold mirror positioned to receive the image light from theimage generator and to reflect at least a first portion of said imagelight in a first reflected direction; a magnifier situated to receivesaid first portion of said image light from said fold mirror, reflectand magnify at least a second portion of said image light combined withenvironmental light transmitted through the magnifier to producecombined light, wherein at least some of said combined light istransmitted through said fold mirror in a second reflected direction tothe eye position of the user, providing an optical path from said imagegenerator to said fold mirror to said magnifier to said eye position ofthe user; and means which redirect light received at normal incidence,for preventing loss of contrast of said image.
 11. Head-mounted visualdisplay apparatus for a user, comprising:an image generator whichoutputs image light, said image generator having a substantially planaroutput surface to output said image light in an image plane, said planaroutput surface being non-orthogonal to the plane of the user's eyes; afold mirror configured to reflect light evenly across the visiblespectrum, positioned to receive the image light from the image generatorand to reflect at least a first portion of said image light in a firstdirection; a reflective combiner situated to receive said first portionof said image light from said fold mirror, reflect at least a secondportion of said image light, combined with environmental lighttransmitted through the reflective combiner to produce combined light,wherein at least some of said combined light is transmitted through saidfold mirror in a second direction to the eye position of the user,providing an optical path from said image generator to said fold mirrorto said combiner, back through said fold mirror and to said eye positionof the user; said image generator and said reflective combiner beingmounted on the head of said user at a level below the top of the head ofsaid user; an optical element positioned between said fold mirror andsaid combiner, wherein said optical element comprises a phase retarder.12. Head-mounted visual display apparatus for a user, comprising:animage generator which outputs image light, said image generator having asubstantially planar output surface to output said image light in animage plane, said planar output surface being non-orthogonal to theplane of the user's eyes; a fold mirror configured to reflect lightevenly across the visible spectrum, positioned to receive the imagelight from the image generator and to reflect at least a first portionof said image light in a first direction; a reflective combiner situatedto receive said first portion of said image light from said fold mirror,reflect at least a second portion of said image light, combined withenvironmental light transmitted through the reflective combiner toproduce combined light, wherein at least some of said combined light istransmitted through said fold mirror in a second direction to the eyeposition of the user, providing an optical path from said imagegenerator to said fold mirror to said combiner, back through said foldmirror and to said eye position of the user; said image generator andsaid reflective combiner being mounted on the head of said user at alevel below the top of the head of said user; wherein at least one ofsaid fold mirror and reflective combiner provides an on-axisconfiguration for said visual display apparatus.
 13. An apparatus, asclaimed in claim 12, wherein said on-axis configuration includes one of:(a) said fold mirror being at an angle of approximately 45° to saidplanar output surface, and (b) said reflective combiner being at anangle of approximately 90° to said planar output surface. 14.Head-mounted visual display apparatus for a user, comprising:an imagegenerator which outputs image light, said image generator having asubstantially planar output surface to output said image light in animage plane, said planar output surface being non-orthogonal to theplane of the user's eyes; a fold mirror configured to reflect lightevenly across the visible spectrum, positioned to receive the imagelight from the image generator and to reflect at least a first portionof said image light in a first direction; a reflective combiner situatedto receive said first portion of said image light from said fold mirror,reflect at least a second portion of said image light, combined withenvironmental light transmitted through the reflective combiner toproduce combined light, wherein at least some of said combined light istransmitted through said fold mirror in a second direction to the eyeposition of the user, providing an optical path from said imagegenerator to said fold mirror to said combiner, back through said foldmirror and to said eye position of the user; said image generator andsaid reflective combiner being mounted on the head of said user at alevel below the top of the head of said user; said fold mirror and saidreflective combiner are used for combining a wavelength of light fromsaid image light with said wavelength of light transmitted through saidreflective combiner.